1 The Object Life Cycle

During the normal operation of a Rails application, objects may be created, updated, and destroyed. Active Record provides hooks into this object life cycle so that you can control your application and its data.

Callbacks allow you to trigger logic before or after an alteration of an object's state.

2 Callbacks Overview

Callbacks are methods that get called at certain moments of an object's life cycle. With callbacks it is possible to write code that will run whenever an Active Record object is created, saved, updated, deleted, validated, or loaded from the database.

2.1 Callback Registration

In order to use the available callbacks, you need to register them. You can implement the callbacks as ordinary methods and use a macro-style class method to register them as callbacks:

It is considered good practice to declare callback methods as protected or private. If left public, they can be called from outside of the model and violate the principle of object encapsulation.

3 Available Callbacks

Here is a list with all the available Active Record callbacks, listed in the same order in which they will get called during the respective operations:

3.1 Creating an Object

before_validation

after_validation

before_save

around_save

before_create

around_create

after_create

after_save

3.2 Updating an Object

before_validation

after_validation

before_save

around_save

before_update

around_update

after_update

after_save

3.3 Destroying an Object

before_destroy

around_destroy

after_destroy

after_save runs both on create and update, but always after the more specific callbacks after_create and after_update, no matter the order in which the macro calls were executed.

3.4 after_initialize and after_find

The after_initialize callback will be called whenever an Active Record object is instantiated, either by directly using new or when a record is loaded from the database. It can be useful to avoid the need to directly override your Active Record initialize method.

The after_find callback will be called whenever Active Record loads a record from the database. after_find is called before after_initialize if both are defined.

The after_initialize and after_find callbacks have no before_* counterparts, but they can be registered just like the other Active Record callbacks.

class User < ActiveRecord::Base
after_initialize do |user|
puts "You have initialized an object!"
end
after_find do |user|
puts "You have found an object!"
end
end
>> User.new
You have initialized an object!
=> #<User id: nil>
>> User.first
You have found an object!
You have initialized an object!
=> #<User id: 1>

3.5 after_touch

The after_touch callback will be called whenever an Active Record object is touched.

4 Running Callbacks

Additionally, the after_find callback is triggered by the following finder methods:

all

first

find

find_by

find_by_*

find_by_*!

find_by_sql

last

The after_initialize callback is triggered every time a new object of the class is initialized.

The find_by_* and find_by_*! methods are dynamic finders generated automatically for every attribute. Learn more about them at the Dynamic finders section

5 Skipping Callbacks

Just as with validations, it is also possible to skip callbacks. These methods should be used with caution, however, because important business rules and application logic may be kept in callbacks. Bypassing them without understanding the potential implications may lead to invalid data.

decrement

decrement_counter

delete

delete_all

increment

increment_counter

toggle

touch

update_column

update_columns

update_all

update_counters

6 Halting Execution

As you start registering new callbacks for your models, they will be queued for execution. This queue will include all your model's validations, the registered callbacks, and the database operation to be executed.

The whole callback chain is wrapped in a transaction. If any before callback method returns exactly false or raises an exception, the execution chain gets halted and a ROLLBACK is issued; after callbacks can only accomplish that by raising an exception.

Raising an arbitrary exception may break code that expects save and its friends not to fail like that. The ActiveRecord::Rollback exception is thought precisely to tell Active Record a rollback is going on. That one is internally captured but not reraised.

7 Relational Callbacks

Callbacks work through model relationships, and can even be defined by them. Suppose an example where a user has many posts. A user's posts should be destroyed if the user is destroyed. Let's add an after_destroy callback to the User model by way of its relationship to the Post model:

8 Conditional Callbacks

As with validations, we can also make the calling of a callback method conditional on the satisfaction of a given predicate. We can do this using the :if and :unless options, which can take a symbol, a string, a Proc or an Array. You may use the :if option when you want to specify under which conditions the callback should be called. If you want to specify the conditions under which the callback should not be called, then you may use the :unless option.

8.1 Using :if and :unless with a Symbol

You can associate the :if and :unless options with a symbol corresponding to the name of a predicate method that will get called right before the callback. When using the :if option, the callback won't be executed if the predicate method returns false; when using the :unless option, the callback won't be executed if the predicate method returns true. This is the most common option. Using this form of registration it is also possible to register several different predicates that should be called to check if the callback should be executed.

9 Callback Classes

Sometimes the callback methods that you'll write will be useful enough to be reused by other models. Active Record makes it possible to create classes that encapsulate the callback methods, so it becomes very easy to reuse them.

Here's an example where we create a class with an after_destroy callback for a PictureFile model:

class PictureFileCallbacks
def after_destroy(picture_file)
if File.exist?(picture_file.filepath)
File.delete(picture_file.filepath)
end
end
end

When declared inside a class, as above, the callback methods will receive the model object as a parameter. We can now use the callback class in the model:

Note that we needed to instantiate a new PictureFileCallbacks object, since we declared our callback as an instance method. This is particularly useful if the callbacks make use of the state of the instantiated object. Often, however, it will make more sense to declare the callbacks as class methods:

class PictureFileCallbacks
def self.after_destroy(picture_file)
if File.exist?(picture_file.filepath)
File.delete(picture_file.filepath)
end
end
end

If the callback method is declared this way, it won't be necessary to instantiate a PictureFileCallbacks object.

You can declare as many callbacks as you want inside your callback classes.

10 Transaction Callbacks

There are two additional callbacks that are triggered by the completion of a database transaction: after_commit and after_rollback. These callbacks are very similar to the after_save callback except that they don't execute until after database changes have either been committed or rolled back. They are most useful when your active record models need to interact with external systems which are not part of the database transaction.

Consider, for example, the previous example where the PictureFile model needs to delete a file after the corresponding record is destroyed. If anything raises an exception after the after_destroy callback is called and the transaction rolls back, the file will have been deleted and the model will be left in an inconsistent state. For example, suppose that picture_file_2 in the code below is not valid and the save! method raises an error.

PictureFile.transaction do
picture_file_1.destroy
picture_file_2.save!
end

the :on option specifies when a callback will be fired. If you
don't supply the :on option the callback will fire for every action.

The after_commit and after_rollback callbacks are guaranteed to be called for all models created, updated, or destroyed within a transaction block. If any exceptions are raised within one of these callbacks, they will be ignored so that they don't interfere with the other callbacks. As such, if your callback code could raise an exception, you'll need to rescue it and handle it appropriately within the callback.

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